1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer, a facsimile, etc.
2. Discussion of the Background Art
Conventionally, in a developing device included in the image forming apparatus, toner is replenished and stirred by a toner-stirring member. Further, a toner amount detection device is provided in the developing device to monitor a replenished amount of toner. Various toner amount-detecting devices that detect such a toner amount have been proposed. For example, the Japanese Patent Application Laid Open No. 2004-46011 discloses a technology capable of detecting a toner remaining amount by detecting a torque applied to a toner-stirring member. However, since toner viscosity changes in accordance with a change of ambient temperature of the image forming apparatus, the detection is hardly accurately performed. Further, the Japanese Patent Application Laid Open No. 9-54488 discloses a toner-stirring device that includes a magnetic sensor that detects a toner remaining amount based on permeability of the toner.
However, the toner-stirring device is used only in a toner hopper, in which toner density does not change. Specifically, when the toner density changes, shortage of the toner can hardly be detected accurately.
Further, the Japanese Patent Application registration No. 3351179 discloses a technology in which a rotation member is pushed and rotated by a toner-stirring member when a developing device is sufficiently replenished with toner. Then, the rotation member drops by gravity and goes ahead of the toner-stirring member to be detected when an amount of remaining toner decreases. Further, a stopper is provided to contact the rotation member plunging below a draft surface of the toner not to be erroneously detected by the toner detection sensor. Because, when the rotation member naturally drops and contacts the stopper, the rotation member decreases its dropping speed and is prevented from plunging under the draft surface of the toner when landing thereon. Thus, the toner draft surface can credibly wait for arrival of an arm of the toner-stirring member. However, since such a technology again employs a magnetic sensor, shortage of remaining toner therein can hardly be detected accurately.
As a technology capable of accurately detecting a toner remaining amount even when toner density changes, a toner empty detection device that employs an optical sensor has been known. For example, the Japanese Patent Application registration No. 2777906 employs a construction, in which a detection objective member is driven rotated and is covered by a detection member cover when toner is sufficiently replenished, and protrudes and is detected by a sensor when an amount of the toner decreases to be short. In such a construction, when the toner is used up, the toner attracts and sticks in a gap between a ring section of a detection member 29 and a shaft 28A inserted into the ring section as shown in FIGS. 3 and 4 in the publication, so that the detection member 29 hardly drops by gravity in relation to the shaft 28A as a rotation center.
In conventional configurations including the above mentioned ones, the rotation member is either detected when it contacts the toner draft surface and its own gravity matches with a resistance of the toner thereby stopping at the toner draft surface or when the toner disappears from a rotational region of the rotation member and the rotation member stops at a bottom dead point. However, in the former configuration, the toner draft surface has unevenness, and the rotation member stops below the toner draft surface more than a little. Since such a phenomenon is affected by many disturbances such as environment of temperature, humidity, etc., a practical performance of the rotation member is unavoidably different from a designing goal. In the latter configuration, since toner massed together on the inner wall surface of the toner container section blocks a movement of the rotation member, toner near end cannot be detected even though only a small amount of toner remains. Otherwise, since the rotation member returns in the reverse direction after passing through the bottom dead point, detection results in erroneous.